Hane seki
Both sides have a string in atari, but for either side, capturing starts a capturing race that the other side can win. Best play is to leave this, therefore it's seki.
Another one
Hane sekis are said to have been discovered by Kaise Takaaki?.
PathsAllYouEverWantedT...
Referenced bySekiOneStoneAliveInSekiDougsGoBlogJune2003